Synthesis and characterization of CdY (Y= Te/O/Se) nanoparticles by wet chemical process

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Synthesis and characterization of CdY

(Y= Te/O/Se) nanoparticles by wet

chemical process

A Thesis Submitted in Fulfillment of the requirements for the Degree of

MAGISTER SCIENTIAE (M.Sc.)

By Kiprotich Sharon

(BSC Hons (Physics))

Student Number: 2013107749

Faculty of Natural and Agricultural Sciences

Department of Physics, University of the Free State (QwaQwa campus),

South Africa, ZA 9866

Supervisor: Prof. F.B Dejene

Co-supervisor: Prof. M.O. Onani

July 2017

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i

Dedication

This thesis is dedicated to my lovely family and my dear

parents.

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ii

Acknowledgements

 Above all I would like to express my most special heartfelt and sincere thanks to

almighty God for His sufficient grace, love, favor and mercy for me. His continuous

renewal of my strength and everlasting guidance helped me to overcome even the most difficult and trying times.

 I would like to express my sincere gratitude to my supervisor, Prof. F.B. Dejene and my co-supervisor, Prof. M.O. Onani who with their advices, constructive criticisms, valuable comments, suggestions, guidance and support throughout the study made my research a success. This study could not have been a success without them.

 I am also thankful to all myUFS QwaQwa campus research colleagues Dr. L.F. Koao,

Dr. K.G. TShabalala, Mr. J. Ungula, Mr. S.J. Motloung, Mr. R. Ocaya, Mr. J.T. Leta, Mr. N. Debelo, Ms M.A. Lephoto, Mr. P.C. Korir and Ms L. Meiki of the

physics department for their continuous support during my period of study.

 I am thankful to the University of Western Cape (UWC) research colleagues (chemistry department); Mr. Ayabei Kiplagat for his unfailing support throughout my synthesis and assisting me access the PL, UV-vis reflection and HRTEM equipment for characterization. Also, Mr. Garvin, Ms. Rose, Mr. Hillary, Ms. Laundrea and Ms.

Zuran for their continuous encouragement and co-operation in helping me complete my

experiments in their laboratories.

 I also want to thank the members of the UFS-QwaQwa campus physics department academic staff for their constant advice on various academic matters relevant to my thesis.

 I am grateful to the UFS QwaQwa campus chemistry department for letting me use their equipment in doing part of the characterization of my samples.

 I want to express my gratitude also to the South African National Research Foundation (NRF) and the University of the Free State for the financial support I benefited from them.

 Lastly, I want to express my indebted appreciation to my beloved family (my husband Jatani and our son Jayden) for their enormous support and our dear parents for their continuous prayers throughout this project. May the Almighty God bless them

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Abstract

Semiconductor quantum dots are nanoparticles with unique tuneable properties. For instance, water soluble nanocrystals which have been synthesized by wet chemistry in open air environment are highly luminescent. They possess well-resolved absorption maxima, high stability and narrow emission bands. This thesis presents several aspects about the synthesis of highly luminescent water soluble, CdTe nanoparticles (NPs) and their near infrared emitting counterpart such as CdTexSe1-x and CdOxTe1-x NPs. It also investigates the synthesis of highly

luminescent NPs specifically engineered to be used for biomedical applications.

Here in a novel approach to synthesize CdY (Y = O/Te/Se) with tunable material properties are presented. The surface morphologies of the as-prepared NPs displayed by SEM micrographs depended strongly on their growth kinetics, probably due to the variation of reaction time, growth temperature or Te concentration. Differences in shape and size were observed depending on the growth conditions. Spherical, rod-like, oval-like and blade-like morphologies were obtained for different reaction parameters. There was observable change in size and shape at longer growth time or higher Te molar ratio. The representative HRTEM analysis showed that the as-obtained CdTe NPs appeared as spherical particles with excellent monodispersity. The images also displayed clear lattice fringes that were an indication of enhanced crystallinity. The X-ray diffraction (XRD) pattern displayed polycrystalline nature of the NPs. XRD pattern confirmed the formation of wurtzite (JCPDS no. 19-0193) and zinc blende (75-2086) phases for the CdYsamples prepared. The type of phase formed depended greatly on the composition, the molar ratios of the consequent elements and reaction conditions of the NPs. The average crystallite sizes estimated from Scherrer equation were found to increase with increase in reaction time, which was in agreement with the HRTEM measurement. The crystallite sizes of the NPs were in the range of 3 to 40 nm depending on the reaction conditions and composition of the NPs formed. Crystallinity of the samples was enhanced up to certain extent as shown by highest peak intensity of the XRD pattern. Variation in the XRD peak intensities was very much dependent on the reaction parameters. Results from XRD also showed a systematic shift in peak positions towards lower and higher 2θ degrees values for CdTe or and CdOXTe1-X NPs,

respectively with an increase/decrease in reaction parameters.

Results from PL showed sharp excitonic band edges of the CdTe, which loses its shoulder during the growth of the NPs. The PL spectra of all the prepared samples indicated a drastic shift in emission window of the core to longer wavelength (500 to 650 nm) which was simultaneously accompanied by variation in emission intensity with different reaction conditions. The position of the emission band was observed to shift towards the lower

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iv wavelength side for shorter durations of synthesis, lower growth temperatures and lower tellurium concentrations. Some difference in absorption edges were observed due to variation in reaction conditions of CdTe NPs. The ultraviolet and visible analysis (UV-Vis) displayed well-resolved absorption maxima which were red shifted upon increase in reaction time, growth temperature and Te concentration. There was an inverse relation between the bandgap and the reaction parameters under study (reaction time, growth temperature and tellurium concentration). The CdTe bulk band gap of 1.5 eV was tuned to even above 3.0 eV while the CdTe counterparts displayed band gap from 1.7 to 2.6 eV.

A pH of 11, reaction temperature of 100 oC and Cd:Te ratio of 1:0.4 were found to be the optimum conditions so far for the preparation of CdY NPs. This method of preparation is simple, sensitive, low cost, easy to execute and efficient with profound advantages such as low reaction temperatures, broad range of pH value and wide PL emission wavelength range thus making it reliable for practical applications.

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Declaration

I (Sharon Kiprotich) declare that the thesis hereby submitted by me for the Master’s degree at The University of the Free State is my own independent work and has not previously been submitted by me at another university/faculty. I furthermore, cede copyright of the thesis in favour of the University of the Free State.

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Key words

 Cadmium Telluride  Nanoparticles  Quantum Dots  Morphology  Excitation  Band Gap  Luminescence  Particle Size  Emission  Absorption Edges  Red Shift  Crystallinity  Lattice Fringes  Growth temperature  X-ray diffraction  Precursor pH  Tellurium  Concentration  Morphology  Crystallite size  Wet-chemical process

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Acronyms

 EDS- Energy Dispersive x-ray Spectroscopy

 PL- Photoluminescence

 SEM- Scanning Electron Microscopy

 TEM- Transmission Electron Microscopy

 XRD- X-Ray Diffraction

 NPs- Nanoparticles

 QDs- Quantum dots

 QWs-Quantum wells

 HRTEM- High resolution transmission electron microscopy

 CIE- Commission Internationale de l’Eclairage

 LEDs- Light emitting diodes

 NIR- near infra-red

 Me- Effective mass of an electron

 Mh- effective mass of a hole

 ICDD- International Centre of Diffraction Data

 UV-vis- Ultraviolet and visible

 JCPDS- Joint Committee on Powder Diffraction Standards

 CdTe- Cadmium telluride

 CdO- cadmium oxide

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Chapter

1

Introduction

1.1 General background

Semiconductor nanoparticles (NPs) or quantum dots (QDs) have drawn a lot of attention recently due to their novel properties such as their size-tunable emission, high photoluminescence (PL) intensity, narrow emission spectra or broad excitation, reasonable photochemical stability, high quantum yields, controllable spectroscopic properties, and the ability to provide specific color fluorescence when excited with a single excitation wavelength and their stability against photo bleaching [1-3]. These properties have been studied for various biomedical and industrial applications such as photonic crystals, light-emitting devices (LEDs), nonlinear optical devices and biological label QDs [4-6]. A QD is a semiconductor core commonly known to be surrounded by a layer of organic ligands. The smallest QDs (< 1 nm in diameters) are nearly molecular (<100 atoms) whereas the largest QDs (>20 nm in size) can be composed of over 100,000 atoms.

The growth of NPs is in the size focusing region when a certain range is considered since smaller particles need less material to grow a shell whereas larger particles need more materials to achieve a shell of the same thickness. When monomer concentration is extremely low, as will always happen when monomers are depleted from the reaction, Ostwald ripening happens. Small particles dissolve to compensate the growth of large particles. Nanoparticles made of metals, semiconductors, or oxides are specifically studied due to their special features regarding their electrical, magnetic, mechanical, chemical, optical and other properties. For further applications nanoparticles (1-20 nm) also referred to as quantum dots have been used in nanomaterial-based catalysts where the nanoparticles act as chemical catalysts. Recently, a range of nanoparticles were extensively investigated for biomedical applications including biolabelling, bioimaging and biosensors [7].

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2 For quantum dots, their emission colors vary depending on the size of the nanoparticles obtained. These nano-sized QDs also show stimulating properties such as an inverse relation of the band gap with the particle sizes, ultra-fast recombination time and PL with high quantum efficiency [8]. Usually, the material properties of nanoregime are considerably different from those of bulk material form. The unique nanomaterial properties are known to be due to the very large surface area -to- volume ratio of the as-prepared NPs (Fig. 1.1) [9, 10].

Surface area increases as volume remain constant

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Figure 1.1 Surface area-to-volume ratio.

There are very few reports for L-cysteine capped CdTe and their near infrared (NIR) emitting counterpart nanocrystals in the biological field. This thesis reports the study of L-cysteine capped CdTe and its counterparts NPs using potassium telluride as stable tellurium source while sodium borohydride was used as a reductant. L-cysteine used as a capping agent can also act as an antioxidant and is widely known to protect cells against oxidative stress and cytotoxicity induced by the QDs. It is also an aminophenol which is friendly to human body since it has no any cytotoxin and it has been considered to be the most suitable stabilizer that promotes the production of spectra which possess very narrow emission and long fluorescence lifetime [11, 12].

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1.2 Basis and scope of this work

1.2.1 Motivation

The main motivation of this research is that growth parameters of CdTe NPs can be controlled and engineered by monitoring the shape, size and size distribution of the as-synthesized NPs among other material features. In biological tissues research, the NIR light travel through and is enfeebled very little due to the fact that the absorbance of the NIR photons by biological tissues is quite low. This implies that penetration of the NIR photons into biological tissues is quite deep, which makes them very useful in biological detection and bioimaging [13]. The various applications of semiconductors are critically reliant on the band gap energy value of the material. Therefore, the ability to tune (or engineer) the band gap is essential for the fabrication of bio-imaging materials with widely varying properties. Selenium being group VI element as sulphur, can easily make an isovalent ternary semiconductor alloys (AB1-xCx). It has also attracted attention because of its antioxidant

properties which yield highly stable nanoparticles [14]. The capability of tailoring material properties promotes production of NPs with various electrical, optical, structural and chemical properties which suit desired applications. Most importantly, is to be able to produce stable and highly crystalline NPs via a facile and less expensive method in large scale. The main objective of this project is to produce NIR emitting NPs because of their vast superior properties.

1.2.2 Statement of the problem

Researchers have been focusing on bridging the gap between the macro, micro and nano regime. This is because the material properties of nanocrystals differ so much from those of the bulk solids. The nanoparticles are also greatly affected by their size and surface chemistry. In the recent years, achievements have mainly focused on the synthesis and characterization of high-quality nanoparticles for various applications in electrical, optoelectronic devices and medication. Researchers have been trying to find ways of using quantum dots inside the human body. Doctors for many years have used dyes to aid the detection of cancer and other ailments. These dyes concentrate in certain parts of the body like where the cancer cells are located. The quantum dots are excited into emitting light that can be detected outside the human body. This light which is detected is turned into a detailed image of tissues which can easily be analysed. Dots of various sizes could be used simultaneously to image different parts of the body because quantum dots emit light at a particular frequency.

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4 Nevertheless, a main concern has been searching for suitable means to produce nanoparticles which are stable and water-soluble. Different kinds of solvents and stabilizers have been studied and used in preparation of the NPs but producing water-soluble NPs has been a major challenge since it required other procedures like ligand exchange in order to make them soluble in water. This research tries to solve the stabilizer and water-solubility problem in that the method used produces highly stable and water-soluble. L-cysteine which if used in this research is human body and environmental friendly. It prevents the NPs from oxidation and associate well with the body since it is part of the proteins found in the human body. Controlling the growth process of CdTe NPs by varying reaction parameters such as reaction time, growth temperature, concentration of the precursor molar ratios and composition is explored in order to form reproducible NPs with desired structure, morphology, size and size distribution for specific optical, chemical, electrical and medical applications. Since the material properties of CdTe NPs can be tailored, NPs of any size and morphology can be produced to emit light at various wavelengths.

1.2.3 Objectives of the study

The specific objectives of the study include:

 To prepare CdY QDs by simple one-pot wet chemical process.

 To characterize and analyse the as-prepared nanoparticles.

 To investigate the influence of reaction times on the morphological, structural and optical properties of the as-prepared samples.

 To study the effect of growth temperature on the morphological, structural and optical properties of the as-prepared samples.

 To study the effect of different tellurium concentration on the morphological, structural and optical properties of the as-prepared samples.

 To estimate the particle sizes of the as-prepared samples with High Resolution Transmission Electron Microscopy (HRTEM).

 To study the surface morphology of the as-prepared samples using scanning electron microscope (SEM).

 To define quality of the as-prepared samples through their chemical compositions using Energy Dispersive X-Ray spectroscopy (EDS).

 To establish the crystal structure and crystallite size with X-Ray Diffraction (XRD).

 To measure the absorption and reflection intensities of the samples and determine their corresponding band gap energies from spectral data by using UV-vis equipment.

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 To study and compare the material properties of CdTe NPs ternary compounds.

 Optimization of the material quality of the as-prepared nanoparticles.

1.4 Thesis Layout

Chapter 1 commences with a general background of nanoparticles outlining their properties, development and presents the basis and scope of this study including the statement of the problem and objectives of this undertaking.

Chapter 2 gives general information on quantum dots, its history over the past decades, gives brief definitions of some common terminologies and applications for the same.

Chapter 3 discusses briefly on the toxicity concerns of QDs used in biomedical applications. Chapter 4 gives short explanation of the experimental equipment/ techniques used to design, synthesize and characterize transition metal compound quantum dots. A summary of various characterization techniques used are also given. This includes a description of the operation of each of the techniques such as UV-vis, PL, XRD, HRTEM, SEM and EDX-S.

In Chapter 5, reports detailed information about the high luminescent L-cysteine capped CdTe quantum dots prepared at different reaction times. This involves the synthesis, analysis and characterisation of CdTe QDs.

In chapter 6, effect of growth temperature on the optical, structural and luminescence properties of CdTe NPs were investigated.

Chapter 7 discusses the effect of tellurium concentration on the optical and structural properties of CdTe NPs were examined using several systems.

Chapter 8 gives a detailed information on a comparison investigation of optical, structural and luminescence properties of CdOxTe1-x and CdTexSe1-x nanoparticles prepared by a simple

one pot method.

List of figures, tables, publications, conferences and possible future suggestions are summarized at the end of chapter 9.

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6

References

[1] W.R. Algar, K. Susumu, J.B. Delehanty, I.L. Medintz, “Semiconductor Quantum Dots in Bioanalysis: Crossing the Valley of Death”, Anal. Chem. 2011, 83, 8826.

[2] M. Gao, C. Lesser, S. Kirstein, H. Mçhwald, A. L. Rogach, H.Weller, “Electroluminescence of different colors from polycation/CdTe nanocrystal self-assembled films”, J. Appl. Phys. 2000, 87, 2297.

[3] N. N. Mamedova, N. A. Kotov, A. L. Rogach, “Albumin−CdTe Nanoparticle Bioconjugates: Preparation, Structure, and Interunit Energy Transfer with Antenna Effect”, J. Studer, Nano Lett. 2001, 1, 281.

[4] F. Chen, D. Gerion, “Fluorescent CdSe/ZnS Nanocrystal−Peptide Conjugates for Long-term, Nontoxic Imaging and Nuclear Targeting in Living Cells”, Nano Lett. 2004, 4, 1827.

[5] F. Fleischhaker, R. Zentel, “Photonic Crystals from Core-Shell Colloids with Incorporated Highly Fluorescent Quantum Dots”, Chem. Mat. 2005, 17, 1346.

[6] H. Huang, A. Dorn, G. Nair, V. Bulovic, M. Bawendi, “Bias-Induced Photoluminescence Quenching of Single Colloidal Quantum Dots Embedded in Organic Semiconductors”, Nano Lett. 2007, 7, 3781.

[7] K.K. Nanda, F.E. Kruis, H. Fissan, M. Acet, “Band-gap tuning of PbS nanoparticles by in-flight sintering of size classified aerosols”, J. Appl. Phys, 2002, 91, 2315.

[8] M.S. Dhlamini, PhD Thesis, University of the Free State, South Africa “Luminescence Properties of Synthesized PbS Nanoparticles”, 2008.

[9] Y. Gogotsi, Nanomaterials Handbook, “Materials Science at the Nanoscale”, Routledge Publishers, USA 2006, 5.

[10] Z. Liu, Y. Liu, J. Zhang, J. Rong, L. Huang, D. Yuan, “Long afterglow in Pr3+ and Li+ co-doped CaZrO3”, Optics Com. 2005, 251, 388.

[11] A.O. Choi, S.J. Cho, J. Desbarats, J. Lovric, D.J. Maysinger, “Quantum dot-induced cell death involves Fas upregulation and lipid peroxidation in human neuroblastoma cells”, Nanobiotechn. 2007, 5, 1.

[12] J. Lovric, H.S. Bazzi, Y. Cuie, G.R.A. Fortin, F.M. Winnik, D.J. Maysinger, “Differences in subcellular distribution and toxicity of green and red emitting CdTe quantum dots”, J. Mol. Med. Heidelberg, Ger., 2005, 83, 377.

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7 [13] P. Yang, M. Ando, T. Taguchi, N. Murase, “Highly Luminescent CdSe/CdxZn1–xS

Quantum Dots with Narrow Spectrum and Widely Tunable Wavelength”, J. Phys. Chem. C, 2011, 115, 14455.

[14] M. Kieliszek, B. Stanisław, “Selenium: Significance, and outlook for supplementation”, Nutrition 2013, 29, 713.

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8

Chapter

2

Literature review on quantum dots

2.1 Brief history on quantum dots

QDs are nano-sized semiconductor particles composed of II-VI group or III-V main group elements. Normally, the size of the quantum dots is between 1 ~ 100 nm. When the electrons and holes within are quantum confined in all the three dimensions, the continuous bandgap structures of the bulk material would become discrete if excited to higher energy states. A photon of a frequency characteristic of that material is emitted when the QDs return to their ground state. This behaviour results in possession of properties intermediate between those of bulk materials and those of discrete molecules.

The history of QDs is known to have begun in the early 1970s with nanometer-thick foils called QWs. Charge carriers (electrons and holes) in QWs become trapped in a few-nanometers-thick layer of wells. The band gap of such QWs is smaller than in the neighbouring barrier layers. Changing the material composition led to the variation of the band gap was achieved in the compound semiconductor. These first low dimensional structures QWs were then followed by invention of quantum wires and QDs. The QD history commenced when Russian physicist Ekimov first discovered the glass crystals in 1980 [1]. Progressive expansion in the science and technology of QDs after 1984 was motivated by derivation of a relation between particle size and bandgap of semiconductor NPs by Luis Brus when he applied the particle in a sphere model approximation to the wave function for bulk semiconductors [2, 3]. Even though it took a long time for new advancement in QDs research, Murray et al. eventually developed QDs with size-tuneable band-edge absorption and emissions [4].

Enormous preparation approaches have been established used to synthesize QD. These methods are divided into two classifications which include the physical approach and chemical approach. The physical approach was commonly known as the epitaxial growth.

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9 However, the main disadvantages of this method are defection formation, size non-uniformity, poor interface quality and even damage to the bulk of the crystal itself. Pyrolysis of organometallic and chalcogen precursors and the main processes that this method relies on where rapid nucleation followed by slower and steady growth is desired [5].

Quantum dots have so many applications in solar cells, light emitting devices, photo bio-chemo labelling technologies because of their enormous reasons: tunable absorbance and emissions, high quantum yields, wide excitation window but narrow Gaussian emission peaks, great opposition to photobleaching when compared to organic dyes, minimal interference and possible functionality with different bio-active agents [6-8].

2.2 Quantum Confinement

The quantum confinement effect causes the optical properties of QDs to evolve dramatically with their size. The quantum confinement effect is described as the phenomenon which causes widening of the bandgap energies of the semiconductor materials when its size is shrunk to nanoscale. The energy required to create an electron and a hole with zero kinetic energy at a distance far enough apart that their Coulombic attraction could be ignored is known as bandgap of that material. If one carrier approaches the other, a bound electron-hole pair (exciton) would is created. This electron-hole pair acts typically like a hydrogen atom, except that a hole, which is not a proton, forms the nucleus. The distance between the electron and hole is referred to as exciton Bohr radius (rB) expressed by the equation below;

𝑟

_𝐵

=

ℏ_𝑒2₂ℇ

(

_𝑚1

𝑒

+

1

𝑚ℎ

)

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The me and mh are the effective masses of electrons and holes, respectively, while ε, ℏ and e

are the dielectric constant, reduced Planck constant and the charge of an electron respectively [9]. If the size of a QD shrinks in such a way that its radius (R) is R < rB, then the material in

question is said to be strongly confined since the motion of the electrons and holes in the QD is confined to the QD dimensions. This size dependent effect was firstly observed in thin films of semiconductor materials (quantum wells) synthesized using molecular beam epitaxy approach [10]. The thickness of the thin film is equivalent to the exciton Bohr radius so that the exciton is confined, which modifies the density of states such that there are fewer band edge states and the bandgap is shifted to the blue [11]. Later studies led to quantum wires/rods and quantum dots. As the QD size is smaller than the material's exciton Bohr radius, the dimensions of the crystal become so small that the photo excited carriers feel the

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10 boundary, causing the continuous density of states in the bulk to collapse into discrete electronic states. In addition, the more confined the carriers are, the higher the bandgap energy is, and correspondingly the potential photoluminescence should blue-shift [12].

2.3 Applications of quantum dots

2.3.1 Solar cells

The ability to produce dots of different sizes allows quantum dots to possess tunable bandgaps across a wide range of energy levels. Semiconductors were used to make traditional solar cells. This however had a disadvantage in that they were expensive to produce. A reported theoretical upper limit value of 33% efficiency is achieved when sunlight is converted to electricity using these solar cells. Tunable band edge offers the possibility to harvest light energy over a wide range of visible-IR light with selectivity. A quantum dot solar cell is a solar cell designed to use QDs as the absorbing PV material. It endeavors to substitute the bulk materials like copper indium gallium selenide (CIGS) or silicon [13].

Figure 2.1: image of cadmium telluride (a) solar cell and (b) photovoltaic cell [14]

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2.3.2 QD-lasers

QD laser structure is fabricated in a way similar to the QW lasers and can be treated like QW lasers in principle. The only main difference arise from the optically active medium where QDs are used instead of QWs. Fig. 2.2 shown below displays a simple laser structure, consisting of an active layer embedded in a waveguide, surrounded by layers of lower refractive index to ensure light confinement. The optically active material medium consists of quantum wells or quantum dots where the bandgap is lower than that of the waveguide material [15].

Figure 2.2: Schematic diagram of a quantum dot lasing device [16]

2.3.3 Indicators

Nanoparticles can be applied in making safety displays for example exit signs which still operate even when there are current blackout cases (Fig. 2.3).

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12 Figure 2.3: (a) Luminescent signs (b) a neon sign and (c) traffic light signs [17, 18]

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2.3.4 Quantum dots LEDs

The main characteristics of QD LEDs are that they consist of pure and saturated emission colours with narrow bandwidth. By changing the size of the QDs, their emission colours or wavelength is easily tuned. They offer high durability and colour purity with high efficiency and flexibility. Moreover, it has an advantage of low processing cost when producing the organic light-emitting devices. Making use of the tunablity of the QDs’ emissions over the entire visible wavelength range from 460 nm (blue) to 650 nm (red) the structure of the QD LED can be engineered. The design of QD-LED is comparable to the structure of O-LED except for fact that the light emitting centres are cadmium telluride nanocrystals in this case. Cadmium-telluride QDs layer is inserted between layers of electron-transporting and hole-transporting organic materials. An applied electric field causes electrons and holes to move into the QD layer, where they are captured in the quantum dot and recombine, and emitting photons. The spectrum of photon emission is narrow, characterized by its full width at half the maximum value. [19, 20]

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2.3.5 Textiles

The field of textiles, focus is given in two main areas; firstly, upgrading existing materials and performances of textiles materials and secondly, developing intelligent textiles with completely new characteristics and functions. The application of nanotechnology in the field of textiles has led to the evolution of nanocompositions, nanofibers, nanopolymers, nanofinishes, etc. In the field of textiles, nanotechnology has been employed in the synthesis of quantum dot (semiconductor nanocrystals). A spray-on coating that mimics the way lotus leaves repel water droplets and particles of dust wasdeveloped by a leading German chemical manufacturer. Lotus plants have super hydrophobia surface and this effect arises because they are coated with hydrophobic wax crystals of around 1 nm (nanometer) in diameter. This lotus effect had led a very significant development in textile technology. The first commercial application of nano technology in textile and clothing industry is found in the form of nano particle (sometimes called nano bead) through a finishing process, which is generally known as nano finishing. One of the developments of nanotextiles is nano fibers [22].

2.3.6 Biological applications

Concerning the QDs in biological applications, two main groups may be discussed; biosensors and labels in biological imaging [23].

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15 A few examples of each group can be seen on the diagram below (Fig. 2.5). For in vivo biological imaging applications of QDs, the fluorescent emission wavelength ideally should be in a region of the spectrum where blood and tissue absorb minimally but still detectable by the instruments. Thus the QDs should emit at approximately 700-900 nm in the NIR region to minimize the problems of indigenous fluorescence of tissues. Moreover, the spectroscopic properties of NIR QDs would allow imaging at a deeper penetration than conventional near-infrared dyes [24].

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References

[1] A.I. Ekimov, and A.A. Onushchenko. “Green synthesis of ZnO nanoparticles via Agathosma betulina natural extract”, Jetp Lett 34, 1981, 345.

[2] L.E. Brus, “Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state” J. chem. Phy. 80, 1984, 4403.

[3] L. Brus, “Electronic wave functions in semiconductor clusters: experiment and theory”, J. Phy. Chem 90, 1986, 2555.

[4] C.B. Murray, D.J. Norris, and M.G. Bawendi. “Synthesis and characterization of nearly monodisperse CdE (E=sulfur, selenium, tellurium) semiconductor nanocrystallites”, J. American Chem Society 115, 1993, 8706.

[5] S.Y. Kaya, E. Karacaoglu, and B. Karasu, “Effect of Al/Sr ratio on the luminescence properties of SrAl 2 O 4: Eu 2+, Dy 3+ phosphors”, Ceramics International 38, 2012,

3701.

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[7] S. Santra, H. Yang, J.T. Stanley, P.H. Holloway, B.M. Moudgil, G. Walter, and R.A. Mericle. “Rapid and effective labelling of brain tissue using TAT-conjugated CdS: Mn/ZnS quantum dots”, Chemical communications 25, 2005, 3144.

[8] H. Yang, S. Santra, G.A. Walter and P.H. Holloway. “Gd III-Functionalized Fluorescent Quantum Dots as Multimodal Imaging Probes”, Advanced Materials 18, 2006, 2890.

[9] P.N. Prasad, Nanophotonics. John Wiley & Sons, 2004.

[10] D.S. Chemla, "Nonlinear optics in quantum-confined structures." Physics Today 46, 1993, 46-52.

[11] D.S. Chemla and D.A.B Miller. “Room-temperature excitonic nonlinear-optical effects in semiconductor quantum-well structures”, JOSA B 2, 1985, 1155.

[12] P. Guyot-Sionnest, M. Hines, “Intraband transitions in semiconductor nanocrystals”, Appl. Phys. Lett. 1998, 72, 686.

[13] Solar cell, [online]. Available from; http://www.solarcellcentral.com/solarpage.html. [Accessed 13 March 2017].

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[14] Quantum dot laser, [online]. Available from;

https://en.wikipedia.org/wiki/Quantum_dot_laser. [Accessed 13 March 2017]. [15] Lasers online. Available from;

http://freshscience.org.au/2010/print-your-own-lasers-lights-and-tv-screens. [Accessed 13 March 2017].

[16] Signs & Tape, [online]. Available from; http://www.glonation.com/signs-and-tape.html [Accessed 17 March 2016].

[17] Neon ‘open’ sign, [online]. Available from http://www.neonclick.de/index.html. [Accessed 10 June 2016].

[18] P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, and V. Bulovic. “Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum”, Nano letters 9, 2009, 2532.

[19] S. Coe, W.K. Woo, M.G. Bawendi, and V. Bulovic. “Electroluminescence from single monolayers of nanocrystals in molecular organic devices”, Nature 420, 2002, 800.

[20] LED applications, [online]. Available from; https://en.wikipedia.org/wiki/Light-emitting_diode. [Accessed 17 June 2016].

[21] Quantum dot textile applications, [online]. Available from;

http://style2designer.com/textile-techniques/nano-textiles/. [Accessed 17 March 2017].

[22] P. Majzlík, J. Prásek, L. Trnková, J. Zehnálek, V. Adam, L. Havel, J. Hubálek, and R. Kizek. “Biosensors for detection of heavy metals”, Listy cukrovarnicke a reparske 126, 2010, 413.

[23] J. Drbohlavova, V. Adam, R. Kizek, and J. Hubalek. “Quantum dots— characterization, preparation and usage in biological systems”, International journal of molecular sciences 10, 2009, 656.

[24] J. Gao, and B. Xu. “Applications of nanomaterials inside cells”, Nano Today 4, 2009, 37.

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18

Chapter

3

Material properties of cadmium telluride

3.1 Outline

Among the group II–VI semiconductor is cadmium telluride (CdTe) with a commonly known Zinc blende/wurtzite crystalline structure, Bohrexciton radius of approximately 6.5 nm with tunable bulk band gap energy corresponding to 1.5 eV at room temperature. They possess superior qualities of long-term stability and brightness which make them ideal candidates for live animal targeting and bioimaging. A study on how to reduce accumulation of NPs in the liver and bone marrow when focusing on long-term imaging of live mice showed the importance of coating NPs with high molecular weight polyethylene glycol (PEG) molecules to [1]. It was discovered that after several months the NPs were still visible in the bone marrow and lymph nodes of the animals which was an indication of high stability of these investigations.Most organic fluorophores used in imaging for extended periods are prone to metabolic and chemical degradation andphotodamage. These make them hard to label cells for extended time. Means to overcome these shortcomings are hunted and the use of genetically encoded organic fluorophores which are constantly prepared and replaced in the cell (L-cystiene) like fluorescent proteins are recommended. After their introduction into cells it takes long period of time before they are detected in the body and cause very long delays when they are faced by photobleaching and other demerits. These shortcomings are minimised when NPs that are resistant to photodamage, degradation by enzymes in live cells and chemical damage are used [2, 3] like CdTe QDs. Thus, NPs have aided the monitoring of molecules in live cells for several hours [4-6], and watching over the cell fate for a whole period of development of an organism [7, 8].

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19

3.2 CdTe crystal structure

The crystal structure of the CdTe nanocrystals formed depend greatly on the growth conditions like the concentration of precursors , growth temperature, reaction time, pH and molar ratios of the precursor elements. The nanocrystals grown had a wurtzite and zinc blende structure depending on the growth conditions. High reaction temperatures favored the formation of zinc blende. Reaction time showed slight changes in crystal structure.

Figure 3.1(a): wurtzite - hexagonal (b) zinc blende - cubic structure of CdTe NPs and (c)

crystal structure displaying the planes and the lattice parameter of the CdTe NPs [9].

3.3 Electronic band structure of CdTe

The CdTe is extensively studied because of the well-known direct band gap energy which ranges from 1.4 to 1.5 eV and can be tuned to a desired value for various applications. The direct materials are differentiated from the indirect gap materials from their relative positions of the conduction band minimum and the valence band maximum in the Brillouin zone. Brillion zone is defined as the volume of k space containing all the values of k up to π/a where a is the unit lattice cell dimension. Both the conduction band minimum and the valence band maximum occur at the zone center where k=0 for a direct gap material while for an indirect gap material the conduction band minimum does not occur at k=0, but rather at some other values of k which is usually at the zone edge or close to it.

(a

)

(b)

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20 Figure 3.2: (a) LDA band structure (b) is schematically experimental band structure of CdTe

and (c) LDA band structure of CdTe obtained with a semi- empirical shift of Cd 4d states and including spin-orbit interaction. The red solid dots denote the s-orbit originated Γ6 state [10].

3.4 Luminescence in CdTe

Figure 3.3: E-k diagrams for a direct band gap material and indirect gap material for the

photoluminescence processes [11].

(c)

(b)

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21 The PL mechanism in semiconductors is graphically demonstrated in Fig. 8, E-k diagrams for a direct band gap material and indirect gap material plots are shown. E and k are the kinetic energy and wave vector (or "momentum vector") of the electron or hole respectively. The absorption of photons energizes the material causing the generation of electrons at the bottom of the conduction band and the holes at the top of the valence band. The movements of the electrons and holes within the conduction and valence bands are due to the rapid thermalization of the excited electrons and holes through phonon emission. The conduction band minimum and the valence band maximum of a direct band gap material (left) occur at the same k values. The momentum of the absorbed or emitted photon is very small (negligible) compared to the momentum of the electron thus the photon absorption and emission (the electron-hole recombination) processes can conserve momentum without the assistance of phonons. On the other hand, an indirect gap material (right) has its conduction band minimum at different k value with the valence band maximum. Therefore, for it to conserve momentum, other processes ere involved for instance the photon absorption process must involve either absorption or emission of a phonon and that the PL process needs the emission of a phonon whose energy (~ 0.01 eV) is much smaller than the energy of the PL photon. The PL peak wavelength approximately gives the band gap of a material.

However, for luminescence to occur in a material a semiconductor structure with a nonzero band gap, Eg should be present in the luminescent material (for example metals do not luminesce because they have no band gap) and that it requires an external source of energy to excite the material before luminescence can occur [11].

3.5 Toxicity in CdTe

The issue of toxicity in QDs presents a major concern when it comes to search of their in vivo application in biomedical imaging. The key source of this toxicity are; the semiconductor materials that commonly constitute the QD core (and sometimes the shell) which can leach under certain circumstances and the generation of reactive and free radical species during excitation [12, 13]. Furthermore, the unique QD nanoscale structure presents a complex set of physiochemical characteristics that further compound any simple studies or conclusions in this direct toxicity issue. As presented in detail in literature, the nanocrystalline cores can be created from various amalgamations of binary semiconductors such as CdTe, InP, CdSe and CdS, just to mention but a few. To further curb the toxicity issue, the cores are normally encapsulated with an ancillary semiconductor material which they are coated with variety of ligands or larger amphiphilic polymers for aqueous compatibility [14, 15]. Moreover, the NPs

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22 can have a wide range of sizes with diameters ranging from 2 nm to greater than 10 nm even though the physical dimensions are on the nanoscale. However, studies done by Ye et al. showed that severe venomousness can be minimised in the as-prepared in-vivo QDs. They demonstrated that the blood and biochemical markers remained within normal ranges following treatment of major organs when rhesus macaques was injected with phospholipid micelle-encapsulated CdSe/CdS/ZnS QDs [16]. No palpable toxicity of QDs and abnormalities observed in mice even at long-time exposure as reported in other biochemical analysis and body weight measurement studies [17]. For biological use, combination of materials and physical properties of QDs can be further modified with either proteins, such as neutravidin, or other biomolecules such as DNA in order to confound any systematic study of toxicity and other issues such as dosage or exposure time can be looked into.

3.6 Absence of cytotoxicity

Extensive research has been conducted concerning the toxicity of QDs to be used for imaging of live cells and organisms for long durations. For such QDs to be used in living organisms it is imperious that QDs are not toxic to cells. Cadmium, tellurium and selenium present in the core of QDs have led to the conviction that the QDs are toxic [18]. However, studies on QDs revealed no toxicity whatsoever in live animals when injected into the bloodstream of pigs and for up to 4 months in mice [1]. No toxicity was detected also when QDs were laden in cells growing in vitro. Furthermore, no trace toxic effects of QDs in cells have been observed in vivo in experiments using Xenopus, Dictyostelium and mouse [19, 20 and 21].

Nevertheless, whenever a new approach for QD synthesis or coating is used, or if QDs are used in an extreme environment that could compromise their integrity, it is important to test for their cytotoxicity. Suitability for multicolor in vivo imaging QDs have facilitated the simultaneous imaging of at least five populations of live cells, each labeled with a different colored QD [22].

Visible light is limited by the fact that they have high absorbance and scatter easily hence hindering imaging in tissue beyond a depth of 100 mm. QDs on the other hand have been found to be orders of magnitude brighter than the conventional fluorescent probes used in multiphoton microscopy. Thus, the use of QDs is highly recommended since they offer significant advantages when a probe requiring simultaneous imaging of multiple fluorophores is conducted. The ability to synthesize QDs that emit in the near-infrared spectrum has also facilitated in vivo imaging not only in mice, but also in bigger animals such as pigs [23-25].

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23

References

[1] B.B. Ballou, C. Lagerholm, L.A. Ernst, M. P. Bruchez, A.S. Waggoner, “Noninvasive Imaging of Quantum Dots in Mice”, Bio conjugate chemistry 15, 2004, 79.

[2] J.K. Jaiswal, M. Hedi, J.M. Matthew, M.S. Sanford, “Long-term multiple color imaging of live cells using quantum dot bioconjugates”, Nature biotechnology 21, 2003, 47.

[3] E.B. Voura, J.K. Jaiswal, M. Hedi, M.S. Sanford, “Tracking metastatic tumor cell extravasation with quantum dot nanocrystals and fluorescence emission-scanning microscopy”, Nature medicine 10, 2004, 993.

[4] D.S. Lidke, P. Nagy, R. Heintzmann, D.J. Arndt-Jovin, J.N. Post, H.E. Grecco, E.A. Jares-Erijman, and T.M. Jovin. "Quantum dot ligands provide new insights into erbB/HER receptor-mediated signal transduction", Nature biotechnology 22, 2004, 198.

[5] M. Dahan, L. Sabine, L. Camilla, R. Philippe, R. Beatrice, T. Antoine, “Diffusion dynamics of glycine receptors revealed by single-quantum dot tracking”, Science 302, 2003, 442.

[6] X. Wu, H. Liu, L. Jianquan, K.N. Haley, A.T. Joseph, J.L. Peter, N. Ge, P. Frank, P.B. Marcel, “Immunofluorescent labeling of cancer marker Her2 and other cellular targets with semiconductor quantum dots”, Nature biotechnology 21, 2003, 41.

[7] B. Dubertret, P. Skourides, D.J. Norris, N. Vincent, A.H. Brivanlou, A. Libchaber, “In vivo imaging of quantum dots encapsulated in phospholipid micelles”, Science 298, 2002, 1759.

[8] A. Hoshino, K. Hanaki, K. Suzuki, K. Yamamoto, “Applications of T-lymphoma labeled with fluorescent quantum dots to cell tracing markers in mouse body”, Biochemical and biophysical research communications 314, 2004, 46.

[9] Crystal structure online; http://en.wikipedia.org/wiki/wurtzite, cubic crystal structure

[Accessed 22 Feb 2017].

[10] Electronic band structure online; http://en.wikipedia.org/wiki/electronic band structure structure [Accessed 23 Feb 2017].

[11] Luminescence online; https://ned.ipac.caltech.edu/level5/Sept03/Li/Li4.html.

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24 [12] E.R. Goldman, E.D. Balighian, H. Mattoussi, M.K. Kuno, J. M. Mauro, P.T. Tran, G.P. Anderson, “Avidin: a natural bridge for quantum dot-antibody conjugates”, J. American Chem Society 124, 2002, 6378.

[13] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, “Semiconductor nanocrystals as fluorescent biological labels”, science 281, 1998, 2013.

[14] D. Gerion, F. Pinaud, S.C. Williams, W.J. Parak, D. Zanchet, S. Weiss, A.P. Alivisatos, “Synthesis and properties of biocompatible water-soluble silica-coated CdSe/ZnS semiconductor quantum dots”, J. Phy Chem B 105, 2001, 8861.

[15] W.C.W. Chan, S. Nie, “Quantum dot bioconjugates for ultrasensitive nonisotopic detection”, Science 281, 1998, 2016.

[16] L. Ye, K. Yong, L. Liu, I. Roy, R. Hu, J. Zhu, H. Cai, “A pilot study in non-human primates shows no adverse response to intravenous injection of quantum dots”, Nature Nanotechnology 7, 2012, 453.

[17] Y. Su, F. Peng, Z. Jiang, Y. Zhong, Y. Lu, X. Jiang, Q. Huang, C. Fan, S. Lee, Y. He, “In vivo distribution, pharmacokinetics, and toxicity of aqueous synthesized cadmium-containing quantum dots”, Biomaterials 32, 2011, 5855.

[18] A.M. Derfus, W.C.W. Chan, S.N. Bhatia, “Probing the cytotoxicity of semiconductor quantum dots”, Nano letters 4, 2004, 11.

[19] D.R. Larson, W. R. Zipfel, R.M. Williams, S.W. Clark, M.P. Bruchez, F.W. Wise, W.W. Webb, “Water-soluble quantum dots for multiphoton fluorescence imaging in vivo”, Science 300, 2003, 1434.

[20] S. Kim, Y.T. Lim, E.G. Soltesz, A.M. De Grand, J. Lee, A.Nakayama, J.A. Parker, “Near-infrared fluorescent type II quantum dots for sentinel lymph node mapping”, Nature biotechnology 22, 2004, 93.

[21] J.A. Kloepfer, R. E. Mielke, M. S. Wong, K. H. Nealson, G. Stucky, J. L. Nadeau, “Quantum dots as strain-and metabolism-specific microbiological labels”, Applied and environmental microbiology 69, 2003, 4205.

[22] L. Zhu, S. Ang, W. Liu, “Quantum dots as a novel immunofluorescent detection system for Cryptosporidium parvum and Giardia lamblia”, Applied and environmental microbiology 70, 2004, 597.

[23] J.K. Jaiswal, S.M. Simon, “Optical monitoring of single cells using quantum dots”, Quantum Dots: Applications in Biology, 2007, 93.

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25 [24] Y.T. Lim, S. Kim, A. Nakayama, N.E. Stott, M.G. Bawendi, J.V. Frangioni, “Selection of quantum dot wavelengths for biomedical assays and imaging”, Molecular imaging 2, 2003, 50.

[25] E.R. Goldman, A.R. Clapp, G.P. Anderson, H.T. Uyeda, J.M. Mauro, I.L. Medintz, H. Mattoussi, “Multiplexed toxin analysis using four colors of quantum dot

fluororeagents”, Analytical Chemistry 76, 2004, 684.

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26

Chapter

4

Characterization techniques of CdY nanoparticles

In this research work, the CdY NPs were characterized using High Resolution Transmission Electron Microscopy (HRTEM), Energy Dispersive x-ray Spectroscopy (EDS), scanning electron microscopy (SEM), X-ray Diffraction (XRD), Photoluminescence spectroscopy (PL) and UV- visible (UV-vis). This chapter describe the characterization techniques in length including how they operate and some key features required for each of the equipment in this chapter.

4.1 Energy Dispersive X-ray Spectroscopy (EDS)

The qualitative analysis (surface chemical or elemental composition) of the as-synthesized nanopowders prepared in this research study was dogged by EDS spectroscopy. EDS or EDX is a chemical microanalysis technique used in conjunction with HRTEM or SEM. X-rays emitted from the sample during bombardment by an electron beam is detected by the EDS technique and sorts out energies of the X-ray for individual elements. Features or phases as small as 1 μm or less can be analysed [1].

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27 The intensity distribution and energy of the signal generated by a focused electron beam impinging on the sample is used to obtain information about the chemical composition of the sample. Electron gun is used as the source of the electron in the high resolution transmission microscope or scanning electron microscope. The incident beam of electrons interacts with core electrons of the sample’s atoms transferring sufficient energy to it, thereby ejecting it from the target atom. This results in the creation of a hole within the atom’s electronic structure. An electron from an outer, higher energy shell then occupies the hole releasing excess energy in the form of an X-ray photon. As a result of electronic transitions which occur between the outer and inner core levels a characteristic X-ray is emitted when the ionized atom ‘relaxes’ to a lower energy state by the transition of an outer-shell electron to the vacancy in the core shell which provide a quantitative and qualitative elemental composition of the sample [3]. Due to a well-defined nature of the various atomic energy levels, it is clear that the energies and associated wavelengths of the set of X-rays will have characteristic values for each of the atomic species present in a sample [4] as shown in Fig. 4.1 (a) and (b).

Figure 4.1(c): Example of an EDS spectrum of CdTe.

A characteristic X-ray is usually emitted when the ionized atom ‘relaxes’ to a lower energy state by the transition of an outer-shell electron to the vacancy in the core shell. The X-ray is called characteristic because its energy equals the energy difference between the two levels involved in the transition and this difference is characteristic of the material.

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28 From the output of an EDS analysis we obtain an EDS spectrum, (Fig 4.1(c)). The EDS spectrum shows the frequency in counts of X-rays received for each energy level. The spectrum normally plots the peaks corresponding to the energy levels for which the most X-rays have been received. Each of these peaks corresponds to a specific atom, and therefore characteristic of a specific element. The intensity of a peak in the spectrum correlates with the concentration of the element in the sample [4].

4.2 X-ray Diffraction (XRD)

X-ray scattering techniques reveal information about the crystallographic structure of nanomaterials and thin films. These techniques are based on observing the scattered intensity of an X-ray beam hitting a sample as a function of incident and scattered angle, polarization, and wavelength or energy. Powder X-ray diffraction (PXRD) is a technique used to characterize the crystallographic structure, crystallite size (grain size), and preferred orientation in polycrystalline, thin films or powdered solid samples.

Figure 4.2(a): An X-ray powder diffractometer [7].

Powder diffraction is commonly used to identify unknown substances. XRD is a crystallographic technique used for identifying and quantifying various crystalline phases present in solid materials and powders. It is a commonly known technique for giving the most definitive crystal structures, interatomic spacing and bond angles. The interference of monochromatic X-rays are produced by a cathode ray tube, filtered to produce

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29 monochromatic radiation collimated to concentrate by heating a filament to produce electrons toward a target by applying a voltage, and bombarding the target material with electrons. A proportion of X-rays are diffracted to produce a pattern. From such a pattern the crystal phases can be identified by comparison to those of internationally recognized databases (such as International Centre of Diffraction Data (ICDD) or the Joint committee on powder diffraction standards (JCPDS)) that contain reference patterns [5]. It is also used for phase identification, determination of grain size, composition of solid solution, lattice constants, and degree of crystallinity in a mixture of amorphous and crystallinesubstances [6]. A sample is said to be crystalline if the atoms are arranged in such a way that their lattice positions are exactly periodic.

Figure 4.2 (b): Schematic diagram of an X-ray tube [7].

Crystalline materials include ceramics, metals, electronic materials, organics and polymers. The X-ray diffractometer are categorised into two broad classes; single crystals and powder. The powder diffractometer, Figure 4.2(a) is routinely used for phase identification and quantitative phase analysis [10]. Electrical current is run through the tungsten filament,

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30 causing it to glow and emit electrons. A large voltage difference (measured in kilovolts) is placed between the cathode and the anode, causing the electrons to move at high velocity from the filament to the anode target.

Upon striking the atoms in the target, the electrons dislodge inner shell electrons resulting in outer shell electrons having to jump to a lower energy shell to replace the dislodged electrons. These electronic transitions results in the generation of X-rays. The X-rays then move through a window in the X-ray tube (Fig 4.2(b)) producing characteristic X-ray spectra which can be used to provide information on the internal arrangement of atoms in crystals or the structure of internal body parts. These X–ray spectra consist of several components and the most common are Kα and Kβ as shown in Fig 4.2(c) [7].

Figure 4.2(c): Characteristic X-ray Radiation [7]

The target materials that are usually used are Cu, Fe, Mo and Cr. Each of these has specific characteristic wavelengths [10] as shown in table 4.1.

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31 Table 4.1: Characteristic wavelengths of target materials [7]

Element Kα Wavelength (λ) Å Mo 0.7107 Cu 1.5418 Co 1.7902 Cr 2.2909

In crystallography, the solid to be characterized by XRD has a space lattice with an ordered three - dimensional distribution (cubic, rhombic, etc.) of atoms. These atoms form a series of parallel planes separated by a distance d, which varies according to the nature of the material. For any crystal, planes have their own specific d-spacing. When a monochromatic X-ray beam with wavelength λ is irradiated onto a crystalline material with spacing d, at an angle θ, diffraction occurs only when the distance travelled by the rays reflected from successive planes differs by an integer number n of wavelengths to produce constructive interference. Such constructive interference patterns only occur when incident angles fulfil the Bragg’s condition such that:

𝑛𝜆 = 2𝑑 sin 𝜃

(2) By varying the angle θ, the Bragg’s Law condition is satisfied for different d-spacing in polycrystalline materials. Plotting the angular positions versus intensities produces a diffraction pattern, which is characteristic of the sample. When a mixture of different phases is present, the resultant diffractogram is a superposition of the individual patterns [8].

In a typical XRD pattern, the diffracted intensities are plotted versus the detector angle 2θ. Each peak is then assigned a label indicating the spacing of a crystal plane. Bragg’s law states the condition for sharp diffraction peaks arising from crystals which are perfectly ordered. Actual diffraction peaks have a finite width resulting from imperfections, either the irradiation source or the sample. A useful phenomenon is that as crystallite dimensions enter the scale the peaks broaden with decreasing crystal size. It is known that the widths of the diffraction peaks allow the determination of crystallite size. Practically, the size of crystallites smaller than 1000 Å can be determined using variants of the Scherer’s equation derived from Bragg’s law:

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32

𝑫 =

𝚱𝝀

𝜷 𝐜𝐨𝐬 𝜽 (3)

Where 𝑫 is the thickness of the crystal, K a constant that depends on the crystallite shape, 𝝀 is theX-ray wavelength (1.54056 Å), 𝜷 is the full width at half maximum of the broadened peak and 𝜽 is Bragg’s diffraction angle. If a Gaussian function is used to describe the broadened peak, then the constant K is equal to 0.89 [9]. XRD has many practical uses for technology enabled sensing applications. Not only does it allow for different phases to be identified, it can also be used to monitor the growth and formation of nanosized crystallites by examining the broadening of peaks in the XRD pattern. This is particularly important for studying sensing materials whose performance depends on the crystal particle size. It is also valuable for determining the distribution of crystals on the surface of a sensing layer [9].

Figure 4.2(d): Bruker D8 Advanced powder Diffractometer

4.3 Photoluminescence Spectroscopy

Luminescence refers to the emission of light by a material through any process other than blackbody radiation [11]. Photoluminescence is luminescence by which electromagnetic radiation, i.e. photons, are used to excite a material, usually by use of ultraviolet light.

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33 Excitation occurs when light is directed onto a sample and it gets absorbed and imparts excess energy into the material. This excess energy can be released by the sample through the emissions of light [12], a process called luminescence. When the luminescence is accompanied by photo-excitation it is called photoluminescence (Fig 4.3 (a)). It is non-destructive because it is based on pure optical processes, no sample preparation is required and it is highly sensitive. Different types of samples (powder, liquid or bulk semiconducting material) can be characterized. PL is a convenient technique requiring a suitable source of optical excitation, a monochromator and a suitable detector for the emitted light. PL measurements are performed under continuous beam excitation conditions commonly known as continuous wave or steady state PL [13]. The sample is optically excited with laser energy greater than its band gap, Fig. 4.3(a). The incident photons are absorbed under creation of electron-hole pairs in the sample. After a short time the electrons eventually recombine with the holes, to emit photons, and light or luminescence will emerge from the sample. The energy of the emitted photons reflects the energy carrier in the sample. The emitted luminescence is collected, and intensity is recorded as a function of the emitted photon energy or wavelength, to produce a PL spectrum.

Figure 4.3(a): Excitation and Emission processes

The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and,

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34 sometimes, quantitative information about chemical composition, structure, impurities, kinetic process and energy transfer. Sensitivity is one of the strengths of the PL technique, allowing very small quantities (Nanograms) or low concentrations (parts-per-trillion) of material to be analysed. Precise quantitative concentration determinations are difficult unless conditions can be carefully controlled, and many applications of PL are primarily qualitative. In PL, a material gains energy by absorbing photon at some wavelength by promoting an electron from a low to a higher energy level. This may be described as making a transition from the ground state to an excited state of an atom or molecule, or from the valence band to the conduction band of a semiconductor crystal or polymer (electron-hole creation). The system then undergoes a non-radiative internal relaxation involving interaction with crystalline or molecular vibrational and rotational modes, and the excited electron moves to a more stable excited level, such as the bottom of the conduction band or the lowest vibrational molecular state. After a characteristic lifetime in the excited state, electron will return to the ground state.

Figure 4.3(b): Cary Eclipse fluorescence spectrophotometer

4.4 Ultraviolet-Visible (UV-vis) absorption Spectroscopy

UV-vis absorption spectroscopy is the measurement of the attenuation of a beam of light after it passes through a sample or after reflection from a sample surface. Ultraviolet and visible

Synthesis and characterization of CdY (Y= Te/O/Se) nanoparticles by wet chemical process